Vehicle body and frame repair shops use various types of pulling devices for applying straightening forces to repair collision damage. The straightening forces applied can be substantial, and often as much as ten tons of force are required to straighten damage to an auto body frame rail, for example. In recent years, unibody frame repair has been a problem because the metal now used is rather brittle. As a result, when applying a straightening force up to about ten tons, the metal is often torn or sheared off under the pulling load.
By way of background, the straightening equipment used in auto body and frame repair shops often comprises a post-type pulling apparatus comprising a vertical load support member with a pulley attached at various elevations along the post. A pulling chain is led over the pulley, and one end of the pulling chain is attached to a portion of the vehicle to which the pulling force is applied. The post also carries a tensioning device, generally a hydraulic cylinder arranged so that the other end of the pulling chain is connected to the hydraulic ram for exerting a pulling force on the pulling chain. In use, the power posts rests on the floor of the body shop, and the post is anchored by an anchor chain extending from the post to at least one anchor pot embedded in the floor on the side of the post opposite from the pulling load. These pulling devices can apply substantial loads in order to straighten collision damage and, hence, create a problem with the brittle metal used in today's unibody cars.
Straightening forces applied by these pulling devices often are applied to a pulling point with the use of a clamp at the end of the pulling chain. The clamp has teeth which bite into the metal when applying the pulling force. These clamps often include a wedge which tightens the jaws of the clamp into the metal workpiece with a progressively greater force in proportion to an increase in the pulling force applied by the pulling chain. When pulling forces approach the ten-ton level, for example, the clamp often bites off the metal in the unibody frame rail before sufficient force can be applied to straighten the frame rail.
Once the metal in the frame rail is sheared off, further problems are encountered because the torn-off frame member must either be replaced, or a new frame part must be welded on to substitute for the torn-off portion of the frame.
Thus, there is a need to provide a means for applying pulling forces of a level sufficient to straighten unibody frame rails while avoiding tearing of the metal in the frame rail under such applied loads.